Compressed air switch with multiple interruption



H. THOMMEN March 9, 1965 COMPRESSED AIR SWITCH WITH MULTIPLEINTERRUPTION 2 Sheets-Sheet l Filed June 15, 1960 INVENTOR ATTORNEYS 2Sheets-Sheet 2 H. THOMMEN March 9, 1965 Filed June 13, 1960 INVENTORHQJLS 7710! men mi -1 www ATTORNEYS United States Patent Oiice 3,172,980Patented Mar. 9, 1965 3,172,9@ CMPRESSED AlR SWITCH WITH MULTTPLEENTERRUPTIUN Hans rllhonnnen, linden, Switzerland, assigner toAktiengesellschaft Brown, lios/eri da Ele, Baden, Switzerland, ajoint-stock company Filed .lune 13, woll, Ser. No. 35,682 Claimspriority, application Switzerland, lune l, i959,

z claims. (ici. 2te- Msi This invention relates in general to compressedair actuated electrical switching apparatus wherein the contacts whichdefine the switching points are blasted with compressed air, and inparticular to switches of the general type described which include aplurality ot power interrupting switching points and a voltagedisconnecting or isolating switching point, all of which are connectedelectrically in series, and one of the power switching points beingparalleled with a low-ohmic resistance. Another of the powerinterrupting switching points can also, if desired, be paralleled by ahigh-ohmic resistance for voltage control. The function of the powerinterrupting switching points is to satisfactorily disconnect the loadcurrent through the switching apparatus up to the highestcircuit-breaking rating for which the apparatus is designed. Thefunction of the voltage disconnecting switching point is to ensurevoltage isolation between the input and output terminals of theswitching apparatus during the entire period that the latter is in itsdisconnected state.

It is known to design air blast switches with a plurality of powerinterrupting switching points wherein such points are associatedtogether in various numbers depending upon the magnitude of theoperating voltage of the system to which the switching apparatus isapplied, and also the magnitude of the load current which must beinterrupted, each of these elements being designed in the same manner.

ln newer developments of air blast switching apparatus, it has ybecomenecessary to further increase their circuit-breaking capacity. To thisend, it has been known to provide two power interrupting switchingpoints in series and to connect a low-ohmic resistance in parallel withone of the power interrupting switching points. A voltage disconnectingswitching point is also connected in series with the two powerinterrupting switching points. ln operation, the power interruptingswitching point which is paralleled by the low-ohmic resistance isopened rst thereby to divert the current flow through the resistance,and then the other power interrupting switching point opens in order tointerrupt the current flowing through the resistance. The voltagedisconnecting switching point is then opened and maintained in an openstate so long as the open circuit condition is desired. The two powerinterrupting switching points can be allowed to re-close as soon as thevoltage disconnecting switching point has been opened so that thecircuit can be re-closed through the switching apparatus simply byre-closing the contacts at the voltage disconnecting switching point.

The compressed air required for actuating the power interrupting andvoltage disconnecting switching points and for blasting the same withthe compressed air in order to facilitate arc extinction across theswitch contacts, is usually obtained from a common supply tank in whichthe air is maintained under the desired pressure. Where the switchingapparatus is composed of a plurality of similar circuit breaking groups,it has already been proposed to sub-divide the pressure tankcorresponding to the number circuit breaking groups, i.e. to associateeach tank with one, or more, groups of power interrupting switchingpoints of `both types, and with one voltage disconnecting switchingpoint. The whole switch assembly thus becomes quite complicated, sinceeach group requires separate air control valves for the switching. Thesupply of compressed air and the arrangement of the air control valvesbecomes even more complicated it the switching motion is different inthe various switching points and if the high-ohmic resistances whenused, also have separate auxiliary switching points to control theirparallel connection with their associated power interrupting switchingpoints, i.e. when the power interrupting switching points for example,close immediately after extinction of the arc, while the voltagedisconnecting switching point remains open, and the auxiliary switchingpoint for the high-ohmic resistances first closes at the instant theentire switching apparatus begins an opening function and then re-opens.

The object of the present invention is to provide an improved structuralarrangement for switching apparatus ot the general type described whichis more simple and economical to manufacture and which permits one tomore easily assemble and adapt the various components to diiierent powerand voltage requirements. In accordance with the invention, the improvedstructural assembly is characterized by two series connected powerinterrupting switching points of the air blast type, one of which isparalleled by a low-ohmic resistance, a voltage disconnecting switchingpoint also of the air blast type in series with the power interruptingswitching points, an auxiliary 'compressed air tank having built-in mainvalves for controlling admission of compressed air to the powerinterrupting switching points, at least one such switching point beingelectrically isolated from the auxiliary compressed air tank by atubular insulator located on the tank.

The invention will become more apparent from the following detaileddescription of several practical embodiments and from the accompanyingdrawings wherein:

FlG. l is a fragmentary view of one embodiment of the invention, theswitching structure essential to an understanding of the invention beingshown partly in vertical central section and partly in elevation;

FlG. 2 is a View similar to FiG. l illustrating a modiiied embodiment ofthe invention;

FIG. 3 is a schematic circuit diagram showing only the arrangement ofthe power interrupting switching points and the voltage disconnecting orisolating switching points in an organization which comprises twosimilar switching units connected in series, each such unit beingconstituted by a series connected arrangement of two power switchin.frpoints and a volta ge disconnecting switching point, and a low ohrnicresistance connected in parallel with a series arrangement of one powerinterrupting switching point of one switch unit and one powerinterrupting switching point of the other switch unit;

FIG. 4- is a view similar to FlG. l but showing a somewhat ditlerentembodiment; and

FIG. 5 is a View similar to FlG. 4 but showing a moditication of thelatter.

With reference now to the embodiment illustrated in FIG. 1 it will beseen that the switch structure comprises an auxiliary compressed airtank l, a laterally extending hollow insulator 2 supported by tank 1 andcontaining a pair of normally closed, power interrupting, switchcontacts 2a, 2b, and a vertically upward extending hollow insulator 3also supported by tank 1 and containing a pair of normally closed, powerinterrupting, switch contacts Ela, 3b. A pair of normally closed,voltage disconnecting, switching contacts 5a, 5b is contained within aninclined hollow insulator 5 which is connected at its upper end with theupper end of insulator 3'. The two power interrupting switching points2a, 2b and 3a, 3b and the voltage disconnecting point 5a, 5b areconnected electrically in series. A low-ohmic resistance 6 is connectedin parallel with the power interrupting switching point 3a, 3b and ahigh-ohmic resistance 4 can, if desired, be connected in parallel withthe power interrupting switching point 2a, 2b.

Within the auxiliary compressed air tank 1 is arranged the main controlvalve structure for the two power interrupting switching points. Thiscomprises a casing 9 containing two compressed air operatedpiston-cylinder type valve units 7 and S disposed at right angles toeach other. Unit 7 functions to control admission of compressed air fromtank 1 into the hollow insulator 2 so as to effect separation of thecontacts 2a, 2b. Unit 8 functions in a similar manner to controladmission ot compressed air from tank 1 into the hollow insulator 3' soas to effect separation of the contacts 3a, 3b.

The auxiliary compressed air tank 1 is supported atop a vertical hollowinsulator column 15 through which compressed air from a suitable primarysource of supply such as a tank (not shown) is introduced into theauxiliary tank 1, and a valve 10 serves to control admission ofcompressed air into casing 9 so as to effect operation of thepiston-cylinder units 7, 8 which, in turn, control admission ofcompressed air to the switch contacts 2a, 2b and 3a, 3b.

The inclined insulator 5' which contains the voltage disconnectingcontacts 5a, 5b is supported by a vertical hollow insulator column 14through which compressed air is fed to the hollow insulator 5 from thesame source which supplies compressed air to the column 15.

The switching apparatus of FIG. 1 operates in the following manner. Whenit is desired to open the cir cuit, valve is opened to admit compressedair from tank 1 to the interior of casing 9. Piston-cylinder valve unit8 functions first to admit compressed air from casing 9 into insulator 3at inlet 12 and elect disconnection of contacts 3a, 3b. This forces thecurrent which had been passing through these contacts when closed totake a parallel path through the low-ohmic resistance 6. Piston-cylindervalve unit 7 then functions to admit compressed air from casing 9 intoinsulator 2 at inlet 11 and effect discon` nection of contacts 2a, 2lbwhich serves to interrupt the current tlowing through resistance 6.After contacts 2a, 2b have been opened, compressed air is admittedthrough insulator column 14 into insulator S to effect separation of thevoltage disconnecting contacts 5a, 5b. These latter contacts aremaintained in their open state by maintaining the air pressure withinthe column 14 and insulator 5 for so long as it is desired to hold thecircuit open at the switching apparatus. However, as soon as thecontacts 5a, 5b have been separated, the power interrupting switchingpoints at contact sets 2a, 2b and 3a, 3b are reclosed. This is effectedby means of the leakage of compressed air through small ports in thepiston elements of the piston-cylinder type valve units 7 and 8 so as toultimately equalize the air pressures on opposite faces of these pistonelements whereupon the restoring forces in springs 16 which are locatedin the cylinders serve to gradually reclose the entrances 11 and 12 tothe insulators 2' and 3' and shut oft the supply of compressed airthereto. A spring loaded blow-off valve structure 13 is located at theouter end of insulator 2 and a similar valve structure is associatedwith the upper end of insulator 3. These valves which are structurallyunited with the movable contacts 2b, 3b are loaded by their springsl insuch manner as to close ott the ends of their respectively associatedinsulators 2', 3 and also move the contacts 2b, 3b into engagement withtheir respective stationary contacts 2a, 3a. When the valves 7, 8 opento admit compressed air into insulators 2', 3', the contacts 2b, 3b areseparated from their respective contacts 2a, 3a and the valves 13 openagainst the counter action of their loading springs to discharge thecompressed air from the insulators. When valves 7, S re-close, theblow-cti valves CII di. 13 also re-close to close ofi the ends of theinsulators and re-close the Contact sets 2a, 2b and 3a, 3b.

The necessary electrical series connection between the stationarycontacts 2a and 3a can be made through the wall of the auxiliary tank llwhich can be made from electrically conductive material. The connectionfrom contact 3b to contact 5b is made by suitable means, not shown indetail.

The embodiment which has been described can constitute one structuralswitch unit of the entire switch assembly. Another such structuralswitch unit containing two power interrupting switching points and avoltage disconnecting point can be arranged to the left of the one shownin FIG. l, i.e. as a mirror image of the first one, in which event thevoltage disconnecting point could be branched ofI from the samesupporting insulator column 14. Alternately a second such structuralswitch unit could be arranged in the same direction as the one depictedin FIG. 1.

FIG. 2 illustrates a somewhat diterent embodiment wherein the powerinterrupting switching point (contact set 2a and 2b within its hollowinsulator structure) is located atop the low-ohmic resistance unitwhich, in this embodiment takes the form of a tubular member 6'supported vertically upon the auxiliary tank 1 and which may besurrounded by an auxiliary hollow insulator. The other powerinterrupting switching point (Contact set 3a and 3b within its hollowinsulator structure) is mounted vertically and directly upon theauxiliary tank 1 parallel with tubular member 6'. The voltagedisconnecting switching point (contact set 5a and 5b within its hollowinsulator structure is likewise mounted directly upon tank 1 and isdisposed in a horizontal attitude laterally of the tank.

in this embodiment, the casing 9 located within tank 1 houses the maincontrol valves '7' and 8. These are similar in structure to the valves7, 8 of the FIG. l embodiment but are actuated, not by compressed airadmitted from tank 1 as in FIG. l, but rather by compressed air suppliedto the casing 9 from a vertical, hollow supporting insulator 14 whichserves as one support for the tank 1'. Air from casing 9 is also fedthrough the horizontal insulator of the voltage disconnecting switchingpoint (contact set 5a', Sb) to separate these contacts after the powerinterrupting switching points (contact sets 2a', 2b' and Sa') have beenopened.

A second hollow supporting insulator 15 is disposed parallel withinsulator -14 and serves as the other support for tank 1. Compressed airwhich feeds upwardly through insulator 15 is admitted to the interior oftank i1 and the latter serves as the supply source of compressed airwhich feeds thnough inlets 11 and 12' when the control valves 7' and`t5' are opened to thus etect disengagement between the powerinterrupting switching points 2a', 2b and 3a', 3b. It will be noted thatthe compressed air which is used to open the contacts 2a', 2b must firstpass upwardly through the tubular low-ohmic resistance unit 6' to thuscool the same. Moreover, the longer air path established by thisresistance uni-t 6 provides the necessary delay in switching sequence sothat the contacts 2a', 2b open subsequently to the opening of contacts3a', 3b. As with the FIG. 1 embodiment, a high-ohmic resistance unit 4can be connected in parallel with the contacts 2a', -Zb for potentialcontrol.

The manner in which the embodiment of FIG. 2 operates is the same asthat of FIG. 1. The advantages of the iFIGS. 1 and 2 embodiments arethat the two power interrupting switching points can be supplied withcompressed air together from a common auxiliary tank, and that theassembly can be effected in a space-saving manner. Moreover, it ispossible to combine any desired number of switch units, depending uponthe voltage to which the switching apparatus is applied and the circuitbreaking capacity desired. Several switch units can be omitted if thecircuit breaking capacity is to be lower. It is even possible toeliminate one power interrupting switching point completely withoutchanging the construction of the individual elements.

The switching for multiple break switches in accordance with theinvention can be etected in any desired order and direction. yForexample, as shown in the circuit diagram of FIG. 3, the number ofnecessary resistances can be reduced and power interrupting switchingpoints of different switch units can be connected together in suchmanner as to be paralleled by a common low-ohrnic resistance unit. Thusthe complete switching assembly is comprised of two series connectedswitching units, each such unit comprising a series arrangement of twopower interrupting switching points 2", 3 and a voltage disconnectingswitching point l5". However, only one low-ohmic resistance unit 6 isused and this is connected in parallel with the two series connectedswitching points 3".

FIGS. 4 and 5 illustrate embodiments wherein the main control valves 17are arranged on the exhaust side of the power interrupting switchingpoints and separation of the contacts at the power interruptingswitching points is effected not by introducing compressed air from anauxiliary tank to the interior of the insulators containing theseswitching points but rather by permitting compressed air which has beenstored in these insulators and auxiliary tank to be dischargedtherefrom.

With reference now to FIG. 4 it will be seen that the auxiliary tank I1is carried on the top of two, parallel spaced hollow insulator columns'14 and 15". Upstanding upon the tank 1 is a tubular insulator -21 whichcontains a set of power interrupting switching contacts 3a, 3b, andmounted at the top of insulator =21 is a housing 19 provided with anexhaust outlet 19a which is controlled by a valve 17. The lower end ofinsulator 21 communicates with the interior of tank 1 and the upper endof insulator 21 communicates with the interior of housing 19. Themovable contacts 3b is loaded by a spring Z3 into engagement with thestationary contacts 3a, and the lov/ohmic resistance 6 is connected inparallel with the contacts 3a, 3b".

Upstanding upon the housing 19 is another tubular insulator 21 whichcontains the other set of power interrupting switching contacts 2a., 2b,and mounted upon the top of insulator 21 is a second housing 19'provided with an exhaust outlet 19a which is controlled by valve 17. Thelower end of insulator 21 is placed in communication with the upper endot insulator -21 by means of a duct 24, and the upper end of insulator21 communicates with housing 19. The movable contacts 2b is loaded by aspring into engagement with the stationary contact 2a".

Also upstanding upon the tank `1" and spaced parallel with theinsulators 21, 21 is another tubular insulator 26 which serves as asupport for the piston elements 22, 22 of the control valves 17, 17 andtheir cylinders and also as a duct for the passage of compressed airfrom the upper end of support insulator 14 to actuate those pistonelements. It will also be noted that compressed air from insulator 14passes into the tubular insulator element which contains the contacts5a", 5b of the voltage disconnecting switching point. As with the otherarrangements, the two power interrupting switching points represented bycontact sets 2a, 2b" and 3a, 3b, and the contact set 5a, Sb are allconnected electrically in series.

The arrangement shown in FIG. 4 operates in the following manner.

When all of the switch contacts are closed, compressed air fills theinsulators 21, 21 and the housings 19, 19'. To ei'ect an opening of theswitch, compressed air is admitted through insulator 14" into thetubular insulator 26. When this air reaches the rst piston 22 it shiftsthe latter to the right thus opening the outlet 19a and permitting thecompressed air stored in housing 19 to flow outward to atmosphere whichresults in an upward movement of contact 3b so as to disengage it fromcontact 3u. The current which had then been flowing through CTL thesecontacts is thus shunted over to flow through the low-ohmic resistance6". By this time, the compressed air ilowing upward through insulator 26past the branch off point to piston 22 will reach the other piston 22 tothus shift it to the right and thus open outlet 19a' which permits thecompressed air stored in housing 19' to how outward to atmosphere andeiect an upward movement of Contact 2b so as to disengage it fromcontact 2a, AS these latter two contacts separate, the current which hadbeen flowing through resistance 6" is interrupted. The flow ofcompressed air into the insulator containing the voltage disconnectingswitching point (contacts Sa, `5b) then causes these latter contacts toseparate, and they remain separated so as to effect the desired voltageisolation so long as the switch is in its disconnected state. However,the pistons 22, 22 are provided with small orifices which ultimatelyserve to equalize the iluid pressure on opposite faces of the pistons sothat the latter are moved back to the left by the action of theirrespective biasing springs 27, 27 and re-close the housing outlets 19aand 19a thereby resulting in a re-closing of the switch contacts 2a", 2band 3a, 3b, and reestablishing the air pressure within the housings 19,19 and insulators 21, 21. When it is desired to reclose the switch, airis vented from insulator 14 t0 atmosphere which results in a re-closingaction of the conitacts Sa, 5b and a venting of air from the tubularinsulator 26.

The series electrical connections between the power interruptingswitching points and the voltage disconnecting switching point can beetlected by making the housing `19, 19 and auxiliary tank 1 from anelectrically conductive material. Thus the circuit through the switchcan be traced from housing 19 to contact 2b" to contact 2a. to housing19 to contact 3b to contact 3a to tank 1" t0 contact 5b to contact 5a".

The arrangement illustrated in FlG. 5 is similar to that of FIG. 4 inthat operation of the switch is eiiected by exhausting compressed airfrom the chamber containing the power interrupting switching contactsand hence similar parts have been given the same reference numerals.However, instead of separate valves for controlling the exhaust from therespective power interrupting switching points, only one control valvestructure is used. With reference now to FiG. 5 it will be seen that atubular insulator 255 upstands upon the auxiliary compressed air tank 1"which is supported by the two, parallel spaced tubular insulators 14 and15". The insulator 2S is lined with a sleeve 29 of conductive materialwhich terminates in the stationary contact 3a" located within one endportion of housing 19 which is supported by the insulator Ztl. Theopposite end of the vertically disposed housing 19 supports an insulator30 through which extends the stationary contact structure Za" that isco-axial with the contact 361'".

The movable contacts 215'" and 3b" are carried by pistons 31, 32respectively which operate vertically in a common cylinder 33 locatedwithin housing 19, these pistons being biased in opposite verticaldirections by a loading spring 3d disposed between the pistons so as tonormally cause the sets of contacts controlled respectively thereby tobe engaged.

Also upstanding upon tank 1 parallel with insulator 2S is anothertubular insulator- 35 through which air is conducted from insulator 14up to the piston 22 of control valve 17 which controls the opening andclosing of the compressed air exhaust outlet 19a from housing 19".Operation of the switch of this embodiment is essentially the same as inthe embodiment of FIG. 4. Cornpressed air introduced through insulator3S causes piston 22 to move to the right thus opening the outlet 19a"and venting the compressed air stored in housing 1% to atmosphere. Thisalso effects a venting of compressed air from the interior of cylinder33 which establishes a temporary pressure differential on pistons 31, 32causing them to move towards each other in succession so as to disengagecontacts Sb" and 211'" from their respective stationary contacts in thatorder. The voltage disconnecting switching contacts Sa and 5b then open.Valve piston 22 then shifts to the left to re-close the outlet 19a"whereupon the contact sets 2517", 2b and 3a", 311'" re-close as thepressure of the air rebuilds within housing 19 and interior of cylinder33.

The series connection of the various switching elements in FIG. 5 can betraced from contact 251'" to contact 2b" to piston 31 and cylinder 33which are made of electrically conductive material to conductive piston32 to contact 3b" to contact 3a" to electrically conductive sleeve 29,to electrically conductive tank 1 to contact 5b to contact 5a.

I claim:

1. A compressed air actuated switch unit comprising a main tankcontaining compressed air, first and second hollow insulator columnsupstanding upon said main tank, an auxiliary compressed air tanksupported atop said iirst hollow insulator for receiving compressed airfrom said main tank, a rst hollow insulator extending laterally fromsaid auxiliary compressed air tank and containing a pair of compressedair actuated power interrupting switching contacts, a second hollowinsulator extending upwardly from said auxiliary compressed air tank andcontaining a second pair of compressed air actuated power interruptingswitching contacts, valve means within said auxiliary compressed airtank for controlling ow of compressed air to said tirst and secondhollow insulators for actuating said first and second pairs of switchingcontacts, a third hollow insulator mounted on said second insulatorcolumn and containing a pair of compressed air actuated voltagedisconnecting switching contacts, one of said voltage disconnectingcontacts being connected to one of the power interrupting contacts ofsaid second pair of such contacts, said irst and second pairs of powerinterrupting switching contacts being connected in series, a low ohmicresistance connected in parallel with said second pair of powerinterrupting switching contacts, and a high ohmic resistance connectedin parallel with said first pair of power interrupting switchingcontacts.

2. A compressed air actuated switch unit comprising a main tankcontaining compressed air, a pair of hollow insulator columns upstandingon said main tank, an auxiliary compressed air tank supported at the topof said insulator columns, the interior of said auxiliary tank being incommunication with said main tank through one of said insulator columns,a low ohmic resistance em- 1bodied in a lirst hollow insulatorupstanding upon said auxiliary tank, a second hollow insulatorupstanding upon said irst hollow insulator and containing a tirst pairof compressed air actuated power interrupting switching contactsparalleled by a high ohmic resistance, a third hollow insulatorupstanding upon said auxiliary tank and containing a second pair ofcompressed air actuated power' interrupting switching contacts and whichare connected in parallel with said low ohrnic resistance, a fourthhollow insulator extending laterally from said auxiliary tank andcontaining a pair of compressed air actuated voltage disconnectingcontacts, all of said pairs of contacts being connected together inseries, and compressed air actuated valve means included within saidauxiliary tank controlling tlow of compressed air into said first,second and third hollow insulators, compressed air to said controlvalves and to said fourth hollow insulator being delivered thereto fromsaid main tank through the other of said insulator columns.

References Cited in the tile of this patent UNITED STATES PATENTS2,453,555 Thommen Nov. 9, 1948 2,599,100 Forwald lune 3, 1952 2,873,331Thommen Feb. 10, 1959 2,894,100 Hoffmann July 7, 1959 2,969,446 FOrwaldlan. 24, l96l FOREIGN PATENTS 568,382 Belgium lune 30, 1958 913,482France May 27, 1946

1. A COMPRESSED AIR ACTUATED SWITCH UNIT COMPRISING A MAIN TANKCONTAINING COMPRESSED AIR, FIRST AND SECOND HOLLOW INSULATOR COLUMNSUPSTANDING UPON SAID MAIN TANK, AN AUXILLIARY COMPRESSED AIR TANKSUPPORTED A TOP SAID FIRST HOLLOW INSULATOR FOR RECEIVING COMPRESSED AIRFROM SAID MAIN TANK, A FIRST HOLLOW INSULATOR EXTENDING LATERALLY FROMSAID AUXILLIARY COMPRESSED AIR TANK AND CONTAINING A PAIR OF COMPRESSEDAIR ACTUATED POWER INTERRUPTING SWITCHING CONTACTS, A SECOND HOLLOWINSULATOR EXTENDING UPWARDLY FROM SAID AUXILLIARY COMPRESSED AIR TANKAND CONTAINING A SECOND PAIR OF COMPRESSED AIR ACTUATED POWERINTERRUPTING SWITCHING CONTACTS, VALVE MEANS WITHIN SAID AUXILIARYCOMPRESSED AIR TANK FOR CONTROLLING FLOW OF COMPRESSED AIR TO SAID FIRSTAND SECOND HOLLOW INSULATORS FOR ACTUATING SAID FIRST AND SECOND PAIRSOF SWITCHING CONTACTS, A THIRD HOLLOW INSULATOR MOUNTED ON SAID SECONDINSULATOR COLUMN AND CONTAINING A PAIR OF COMPRESSED AIR ACTUATEDVOLTAGE DISCONNECTING SWITCHING CONTACTS, ONE OF SAID VOLTAGEDISCONNECTING CONTACTS BEING CONNECTED TO ONE OF THE POWER INTERRUPTINGCONTACTS OF SAID SECOND PAIR OF SUCH CONTACTS, SAID FIRST AND SECONDPAIRS OF POWER INTERRUPTING SWITCHING CONTACTS BEING CONNECTED INSERIES, A LOW OHMIC RESISTANCE CONNECTED IN PARALLEL WITH SAID SECONDPAIR OF POWER INTERRUPTING SWITCHING CONTACTS, AND A HIGH OHMICRESISTANCE CONNECTED IN PARALLEL WITH SAID FIRST PAIR OF POWERINTERRUPTING SWITCHING CONTACTS.